1. Field of the Invention
The present invention relates to a magnetic field generator for devices utilizing magneto-optical effect, an optical device and optical attenuator which incorporate such a magnetic field generator, and a method of fabricating a base substrate for the magnetic field generator. More particularly, the present invention relates to a magnetic field generator which applies an arbitrary magnetic field distribution to a magneto-optical crystal, as well as to an optical device and optical attenuator incorporating such a magnetic field generator. It further relates to a method of fabricating a base substrate for that generator.
2. Description of the Related Art
Strenuous efforts have been made these days to develop high-bandwidth, high-speed data communications networks to meet the needs for realtime distribution of large amounts of data, including high-quality images and videos. Particularly, the use of the Internet is continuously expanding, and this situation raises an issue of how to handle the rapidly increasing network traffic. One approach is to increase the number of information-carrying channels that are multiplexed in a fiber optic cable. While there are several ways to achieve this, the wavelength-division multiplexing (WDM) is known as an especially promising technology for high-bandwidth data transport. WDM systems enable us to send multiple optical signals with different wavelengths over a single fiber, and they have actually been deployed in long-haul telecommunications network infrastructures.
In such long-haul optical networks, optical amplifiers should be placed midway to compensate for fiber loss. Some amplifies perform optical-to-electrical conversion to amplify the signal in electrical form, while others boost the intensity of optical signals all optically. The latter type is of greater interest these days because they can be implemented at lower costs than the former type.
The all-optical amplifiers, however, exhibit some non-linear response to different wavelengths. When a plurality of such amplifiers are deployed on an optical path, the spectral distribution of a transmitted optical signal would be seriously distorted when it arrives at the receiving end. Further, increased crosstalk noise is another problem that is caused by the non-linearity of optical amplifiers. It is difficult to receive the information without solving those problems.
The above-described difficulties with conventional optical amplifiers come from their wavelength-dependent gain characteristics. This is called xe2x80x9cgain tiltxe2x80x9d in optical communications terminology, which is one of the negative factors that limit the maximum transmission distance of a WDM system. In order to reduce the wavelength dependence of amplifier gains, an optical channel equalizer is inserted in the WDM transmission line, which splits a given WDM signal into individual wavelength components (i.e., into individual channels), gives an appropriate attenuation to each channel, and recombines them into a single optical beam for transmission. To this end, conventional systems employ a plurality of optical attenuators. Such systems, however, need as many attenuator modules as the number of WDM channels, which increases the size and complexity of network equipment.
As a solution for the above problem, one of the inventors of the present invention proposed a variable optical attenuator in the Unexamined Japanese Patent Publication No. 11-119178 (1999), which is the basis of the U.S. Pat. No. 5,999,305 granted to the same inventor. The proposed attenuator uses magneto-optical effect to yield a desired attenuation profile for multiple-channel optical signals. More specifically, a magneto-optical crystal is combined with a means for exposing it in a magnetic field with an arbitrary distribution. This single optical device can provide arbitrary attenuation to each individual optical channel.
FIG. 17 shows the concept of the conventional variable optical attenuator mentioned above. A given WDM signal runs through an optical fiber 410 until it reaches two dispersion devices (gratings) 420 and 430, where the light is split into individual wavelength components dispersed in the X-axis direction. The resulting parallel rays of light are incident on a magneto-optical crystal 455 with a reflective coating 456 on its back. The rays are reflected at the reflective coating 456, and the returning light goes back through the same optical path as described above.
The magneto-optical crystal 455 is disposed between permanent magnets 457a (S pole) and 457b (N pole), so that magnetic saturation will be reached in that crystal 455. The magneto-optical crystal 455 is further applied a controlled magnetic field generated from an array of main magnetic cores 454. Here, we can produce any desired magnetic field distribution by commanding a controller 460 to vary electrical current of each individual main magnetic core 454. The magneto-optical crystal 455 serves as a Faraday rotator, which changes the polarization angle of each optical signal component under the influence of the magnetic field being applied. The Faraday rotation angle of a particular wavelength component is determined by the magnetic field strength at a corresponding portion of the magneto-optical crystal 455. A birefringent crystal 440 is placed on the optical path, so that the optical signal will be attenuated in accordance with that Faraday rotation angle. The mechanism of FIG. 17 gives an arbitrary attenuation level to each different wavelength channel in this way.
While the above-mentioned patent application provide almost no details as to the structure of the magnetic field generator 450, there are a couple of other literatures that analogously suggest how to construct it. Although they are originally designed, not for optical attenuators, but for use in a magnetic display device, we are now going to present those two prior-art examples. Both of them are magnetic write heads that apply vertical magnetic fields on a magnetic display medium.
Referring to FIG. 13, a first example of such a conventional magnetic head unit is shown. According to the disclosure in the Unexamined Japanese Patent Publication No. 8-167112 (1996), the body of this unit comprises a flexible circuit board 216 and a housing plate (holding member) 204 made of non-magnetic material. Processed on the housing plate 204 are a plurality of housing cavities 210 each having a side slit 212. The housing cavities 210 accommodate a plurality of discrete coil units, each being composed of a magnetic core 208 made of magnetic material and a coil 206 with terminals 214a and 214b. While FIG. 13 shows them separately, the coil 206 is actually wound around the magnetic core 208.
Every housing cavity 210 has an opening at the front end of the housing plate 204 and a side slit 212 on the top surface of the same. The air-core coils 206 are inserted through the front openings, together with the magnetic cores 208, one for each. The terminals 214a and 214b of each coil 206 are guided out of the housing cavity 210 through the slit 212 and through-holes 217 on the flexible circuit board 216. Finally, they are connected electrically (e.g., by soldering) to some conductors on the flexible circuit board 216, which provides wiring to coil driver circuits (not shown). The coils 206 are energized by individual drive currents that are supplied through the wiring on the flexible circuit board 216, whereby a desired magnetic field is produced in each corresponding magnetic core 208.
Another example of a conventional magnetic head unit is shown in the Unexamined Japanese Patent Publication No. 11-219507 (1999). FIG. 14 depicts the structure of this second example, and FIG. 15 is an enlarged cross-sectional view of part C of FIG. 14. The illustrated magnetic write head has a plurality of very thin coil units 320 that are arranged side by side on a single plane. More specifically, it is constructed with the following components: a flexible circuit board 304, electrically non-conductive substrates 311 and 312, terminals 313, common electrodes 314, individual electrodes 315, and the coil units 320.
An array of the coil units 320 are aligned along one end face 311a of the substrates 311 and 312. Each coil unit 320 has a thin long magnetic layer 323 serving as a core, and surrounding conductive layers 321, 322, 324, and 325 form a coil winding around the magnetic layer 323, spanning its length. Such a coil unit array is sandwiched by two electrically non-conductive substrates 311 and 312. Fabricated on the lower substrate 311 are: the terminals 313, common electrode 314, and individual electrodes 315. All coil units 320 are connected to the common electrodes 314 at their one end, and these common electrodes 314 reach two terminals 313 near the edges of the substrate 311. The remaining ends of the coils are connected to their corresponding individual electrodes 315, and those individual electrodes 315 are wired to the remaining terminals 313 individually. The flexible circuit board 304 is bonded onto the non-conductive substrate 311 in such a way that its printed conductors are in contact with the terminals 313. With the arrangement described above, each coil unit 320 produces a magnetic field H that is substantially perpendicular to the end face 311a of the non-conductive substrate 311.
Referring to FIG. 16, the manufacturing process of the above-described coil units 320 will be described below. The process begins with evaporating copper onto a non-conductive substrate 311. The deposited copper film is then subjected to an etching process to form predefined patterns as shown in part (a) of FIG. 16. This will be a bottom layer 321 of the intended rectangular coil. Although not shown in FIG. 16, the terminals, common electrodes, and individual electrodes are fabricated also at this stage of the process. In the next step, the coil layer 321 is coated with an insulating plastic material by using a screen printing technique. The resulting lower insulation layer 326 is shown in part (b) of FIG. 16. Subsequently, a magnetic layer 323 is formed with a predetermined mask pattern by evaporating Fexe2x80x94Ni on the insulation layer 326 as shown in part (c) of FIG. 16. In the step shown in part (d) of FIG. 16, the top and both sides of the magnetic layer 323 are coated with a plastic material by using a screen printing technique, which forms insulation layers 327 to 329. In this way, the magnetic layer 323 is fully covered with insulating material.
The above process is followed by electroless copper plating. Here, copper is deposited on the exposed side edges of the bottom coil layer 321, so that the insulation layers 326 to 329 will be sandwiched by the resulting copper walls as shown in part (e) of FIG. 16. Additional coil layers 324 and 325 have thus been produced as two side walls of the intended rectangular coil. The next step shown in part (f) of FIG. 16 is to evaporate copper on top of the insulation layers 327 to 329 and etch the deposited copper film with an appropriate mask pattern that produces an electrical connection with the two side coil layers 324 and 325. An upper coil layer 322 is formed in this way, meaning the completion of individual coil units 320. Then, every open space between coil units is filled with insulating plastic material as shown in part (g) of FIG. 16, which is referred to as a filling layer 330. After that, an upper non-conductive substrate 312 is placed and fixed on the coil units 320, bringing the magnetic write head to completion.
Recall here that we need a magnetic field generator suitable for use in optical devices such as those discussed in FIG. 17. The magnetic field generator 450 in FIG. 17 is supposed to provide the magneto-optical crystal 455 with an arbitrary distribution of magnetic field, and that distribution has to be continuous in the X-axis direction and uniform in the Y-axis direction at least over the width (swing width) of the light beam incident on the magneto-optical crystal 455. Furthermore, it is a crucial requirement that the magnetic field generator 450 be small in size and low in cost, besides being stable and reliable in operation.
The structure of conventional magnetic head units, however, fail to satisfy the above-noted requirements because of their disadvantages described below. Take the first conventional magnetic head unit discussed in FIG. 13, for example. This unit is constructed as a linear array of discrete coil units arranged in parallel with the slits 212, each coil unit having been separately assembled from a magnetic core 208 and a coil 206. Such discrete coil units are not suitable for mass-production or cost reduction, because it is a labor-intensive task to wind a coil 206 around each magnetic core 208 and soldering each coil""s terminals 214a and 214b. 
Further, in order to realize a smaller magnetic head unit, it is necessary to shrink the magnetic cores and coils and reduce the pitch between coil units. This will enviably lead to the use of smaller-gauge wire for the coils, reduction of their number of turns, and use of thinner magnetic cores. All those factors make the manufacturing process much more difficult. We would only end up with the problems of insufficient accuracy in machining and assembling, reduced mechanical strength of components, and increased cost. After all, the use of discrete coil components has an inherent limitation when it comes to unit size reduction.
Now think of the second conventional magnetic head unit discussed in FIGS. 14 to 16. Unlike the preceding one, this unit is designed to use thin-film and thick-film technologies to process a magnetic layer 323, insulation layers 326 to 329, coil layers 321, 322, 324, and 325. This approach of using fine fabrication techniques is certainly advantageous in that smaller coil units 320 can be constructed in high accuracy and repeatability. There is one problem, however; the thickness of the magnetic core (magnetic layer 323) is limited by the performance of film forming processes. Actually, the thickness is a few tens of micrometers (xcexcm) at most. For this reason, the generated magnetic field has a width of several tens xcexcm in the Y-axis direction, while being sufficiently broad and continuous in the X-axis direction. On the other hand, a light beam from an optical fiber is collimated with a lens, resulting in a parallel light beam, typically of several hundreds of xcexcm (up to 500 xcexcm) in width. The magnetic layer must be sufficiently thick to cover this light beam width, but it is very difficult to form such a thick magnetic layer with the film-forming process used in the second conventional unit.
In addition, the above two conventional units have a common deficiency; they lack a closed path of magnetic field. Both units have no extra magnetic objects that may guide the magnetic flux generated by coil units and form a closed magnetic circuit. This means that the generated magnetic field is likely to disperse in different directions. When a plurality of coil units are tightly arranged, a magnetic field emanating from one magnetized core may reach the adjacent coil units via the target device, causing an unwanted cross-talk between closely placed coil units. Besides introducing instability to the operation, the lack of appropriate magnetic paths would make the unit less resilient to external disturbances.
In view of the foregoing, it is an object of the present invention to provide a magnetic field generator which is small in size, capable to producing optimal magnetic field distribution with reduced interference between adjacent cores, and suitable for mass production.
Another object of the present invention is to provide a method of fabricating a base substrate for the magnetic field generator described above.
Yet another object of the present invention is to provide a variable optical attenuator which takes advantage of the magnetic field generator described above.
Still another object of the present invention is to provide an optical device which takes advantage of the magnetic field generator and variable optical attenuator described above.
To accomplish the first object, the present invention provides a magnetic field generator which produces a magnetic field for controlling attenuation of an optical signal. This magnetic field generator comprises the following elements: an insulating substrate made of non-magnetic material, having a plurality of hollows; a plurality of coils formed on the insulating substrate in alignment with the hollows, each having at least one layer of conductive film shaped in a spiral pattern; a plurality of main magnetic cores made of magnetic material, each of which is mounted on the insulating substrate, partly fitting into the hollows; a target device held on the insulating substrate, which is exposed in a combined magnetic field produced by the plurality of main magnetic cores; and a permanent magnet held on the insulating substrate, which applies a magnetic field to the target device so as to bring about magnetic saturation thereof.
In addition, to accomplish the second object, the present invention provides a method of fabricating a base substrate for a magnetic field generator, including coils formed on a silicon substrate and a structure to hold magnetic objects. This method comprises the following steps: (a) forming an insulation film on top and bottom sides of the silicon substrate, and partly removing the bottom-side insulation film to create an opening at a place where a hollow will be made; (b) etching the silicon substrate using the bottom-side insulation film as an etching mask to produce a hollow as deep as the thickness of the silicon substrate, whereby the top-side insulation film remains unetched at the bottom of the produced hollow; (c) forming a conductive thin film in a spiral pattern on the top side of the insulated silicon substrate, stacking an insulating thin film thereon, and partly removing the insulating thin film to create openings for coil terminals, electric contacts, and upper hollows; (d) forming another layer of the conductive thin film in a spiral pattern, stacking another layer of the insulating thin film thereon, and partly removing the insulating thin film to create openings for the coil terminals and upper hollows; and (e) removing the top-side insulation layer remaining in the hollows.
Further, to accomplish the third object, the present invention provides a variable optical attenuator which receives a multiplexed optical signal and outputs the same after attenuating each optical component contained therein. This variable optical attenuator comprises the following element: a lens which turns a given optical signal into a collimated beam; a first dispersion device which causes spectral dispersion of the collimated beam to obtain individual wavelength components thereof; a second dispersion device which renders the individual wavelength components parallel, thereby producing a parallel light beam; a birefringent crystal which causes polarization splitting of the parallel light beam; and a magnetic field generator which applies a magnetic field to the parallel light beam has passed through the birefringent crystal. Here, the magnetic field generator comprises the following elements: an insulating substrate made of non-magnetic material, having a plurality of hollows; a plurality of coils formed on the insulating substrate in alignment with the hollows, each having at least one layer of conductive film shaped in a spiral pattern; a plurality of main magnetic cores made of magnetic material, each of which is mounted on the insulating substrate, partly fitting into the hollows; a target device held on the insulating substrate, which is exposed in a combined magnetic field produced by the plurality of main magnetic cores; and a permanent magnet held on the insulating substrate, which applies a magnetic field to the target device so as to bring about magnetic saturation thereof.
Moreover, to accomplish the fourth object, the present invention provides an optical device which equalizes the intensity of different wavelength components contained in a multiplexed optical signal. This optical device comprises the following elements: (a) an optical fiber cable carrying the multiplexed optical signal; (b) an optical amplifier which amplifies the multiplexed optical signal; (c) an optical coupler which splits a fraction of the multiplexed optical signal that is amplified by the optical amplifier; (d) a multi-channel optical monitor, coupled to the optical coupler, which measures the intensity of each spectral component contained in the fraction of the multiplexed optical signal; (e) a controller, coupled to the multi-channel optical monitor, which produces electrical signals according to the measured intensity of each spectral component; (f) an optical circulator having a first to third ports to route one optical signal from the first port to the second port and another optical signal from the second port to the third port, the first port being coupled to the optical coupler; (g) a variable optical attenuator which gives an attenuation to each spectral component of the multiplexed optical signal received from the second port of the optical circulator, as specified by the electrical signals supplied from the controller, and sends the resulting equalized optical signal back to the second port of the optical circulator. Here, the variable optical attenuator comprises the following element: (g1) a lens which turns a given optical signal into a collimated beam; (g2) a first dispersion device which causes spectral dispersion of the collimated optical signal to obtain individual wavelength components thereof; (g3) a second dispersion device which renders the individual wavelength components parallel, thereby producing a parallel light beam; (g4) a birefringent crystal which causes polarization splitting of the parallel light beam; and (g5) a magnetic field generator which applies a magnetic field to the parallel light beam that has passed through the birefringent crystal. Further, the magnetic field generator comprises the following elements: (g5a) an insulating substrate made of non-magnetic material, having a plurality of hollows; (g5b) a plurality of coils formed on the insulating substrate in alignment with the hollows, each having at least one layer of conductive film shaped in a spiral pattern; (g5c) a plurality of main magnetic cores made of magnetic material, each of which is mounted on the insulating substrate, partly fitting into the hollows; (g5d) a target device held on the insulating substrate, which is exposed in a combined magnetic field produced by the plurality of main magnetic cores; and (g5e) a permanent magnet held on the insulating substrate, which applies a magnetic field to the target device so as to bring about magnetic saturation thereof.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.